Came across this problem today. I logged into the site an noticed that there was an update available to a plug in that I use on the site. I went to update and it looked like it went OK and updated successfully. NO!
About 5 minutes later I went to pop a new article on the site only to find the I got a message saying “WordPress “Briefly unavailable for scheduled maintenance”. Here we go I thought. After a bit of research I find that when
you do any sort of upgrade to WordPress it puts a file into the root of your site called .maintenance. This forces all visitors to your site to see the notice of “Briefly unavailable for scheduled maintenance”. We need to remove this file to get the blog running again.
To do this either log into dreamweaver and go to your site in remote view and remove the .maintenance file. If you dont use dreamweaver use either your FTP client or if you are hosted on cpanel you can also gain access to your root folder from there. Once removed you will find your site loads up fine.